Modern gas turbine engines operate at high rotational speeds and high temperatures for increased performance and efficiency. Thus, the materials from which gas turbine engine components are made of must be able to withstand this severe operating environment.
Most high temperature gas turbine engine components are made of nickel base superalloys which are alloys specifically developed for high temperature and high mechanical stress applications. Superalloys are often cast into the component shape. For example, directional solidification is known in the art. This casting technique aligns grain boundaries parallel to the stress axis. This alignment enhances elevated temperature strength. Directional solidification aligns the grains to minimize failure initiation sites because high temperature failure usually occurs at the boundaries between metal crystals.
An extension of the above technique is single crystal casting. Casting of alloys in single crystal form eliminates internal crystal boundaries in the finished component. Single crystal turbine blades and vanes possess superior characteristics such as strength, ductility, and crack resistance at high operation temperatures. Thus, single crystal components are extensively used in the turbine section of gas turbine engines.
Although single crystal engine components are desirable, they are extremely costly to manufacture. Defects often occur during manufacturing, as well as after extensive engine operation. Upon detection of cracks or other defects the component must be repaired or replaced. There is currently no effective repair method capable of restoring a single crystal component to its original crystal morphology and properties. This incurs a significant expense on the industry.
The successful use of conventional techniques for repairing metallic single crystal components has not been accomplished because of inadvertent crystal boundary introduction and solidification cracking. Many welding type processes have been characterized as providing "single crystal repair" yet these processes invariably introduce boundaries, resulting in reduced mechanical properties.
Most prior art repair processes have, to our knowledge, employed finely focused laser beams of high power density to interact with the metal substrate. The result has been cracking due to at least two phenomena. The first phenomena relates to a high rate of solidification. The high rate of solidification results from the high temperature difference between the laser beam created molten pool and the substrate. This temperature difference is a consequence of the rapid heating rate which does not permit the unmelted substrate to achieve any significantly elevated temperature. This means that when the laser beam moves on or is shut off, the melted surface portion will rapidly solidify because the substrate acts as an extremely effective heat sink.
More specifically, the high power densities and short exposure times lead to high thermal gradients and high cooling rates which result in rapid solidification rates. This type of localized melting and solidification can induce thermal stresses-during solidification which can lead to cracking.
The second phenomena which leads to cracking and which results from many prior art teachings is that the pool is deep and has a high aspect ratio (depth to width). In the solidification of such a relatively narrow deep molten pool, several adverse effects occur. For example, the heat flow will be sideways from the pool as well as down into the substrate because of the relatively high ratio of depth to width. As the solidification reaches a conclusion, there will be a high state of stress resulting from the constraint of the pool walls. The net effect of a high ratio is the introduction of high angle grain boundaries and a heavily constrained solidification condition. Introduction of high angle grain boundaries reduces the integrity of the material and increases the susceptibility to cracking. The high constraint of this type of solidification leads to high stresses during and after solidification which can also lead to cracking. Thus, for the previously enumerated reasons prior art laser metal treatment techniques have been prone to cracking and have generally been difficult to use.
There have been attempts to alleviate some of these problems. These attempts include preheating the substrate to reduce cracking as well as the use of different filler materials, such as filler materials having more ductility and less of a propensity for solidification cracking. Unfortunately, these attempts to solve the problem have been relatively unsuccessful.
Accordingly, there is a need for a method of repairing a single crystal metallic article such that the single crystal character of the article is maintained.